J. Org.Chem. 1988,53, 4827-4831
4827
Acceleration of the Pseudorotation Rates in Pentacoordinated Phosphorus Compounds. Conformational Transmission versus Hexacoordinated Zwitterionic Intermediates A. E. H. de Keijzer* and H. M. Buck Department of Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 M B Eindhoven, The Netherlands Received March 29, 1988
A variable-temperature 13C NMR study, accompanied by a high-resolution 'H NMR conformational analysis study, on a series of monocyclic oxyphosphoranes is reported. The selected compounds enabled us to study the acceleration of the rates of intramolecular ligand reorganization on pentacoordinated phosphorus. It allowed us t o determine whether the enhancement of the reorganization rates was brought about by accelerated pseudorotation due t o the conformational transmission effect or by the involvement of hexacoordinated zwitterionic phosphorus intermediates. The results of the study further substantiate the findings that the involvement of such hexacoordinated intermediates is of no importance in the type of oxyphosphoranes studied.
The concept of conformational transmission in pentacoordinated (P(V)) trigonal-bipyramidal (TBP) phosphorus compounds has received considerable attention during the past few years.' In these studies it has been shown that phosphorus compounds possessing the common PO-C-C-O fragment are subject to a conformational rearrangement around the central C-C linkage of this fragment if the coordination state of phosphorus is increased from four (P(1V)) to five (P(V)-TBP). The incorporation of an additional ligand in a P(1V) geometry causes a considerable change in the intrinsic chemical bonding properties around the central phosphorus atom,2 resulting in an enhanced electron density on the axially located oxygens linked directly to phosphorus. In its turn, this effect is transmitted into a conformational change around the central C-C linkage of the axially located 0-C-C-0 fragment. The actual conformation of the 0-C-C-0 linkage is changed from the well-known gauche orientation in the P(1V) state to a pronounced anti orientation of the two vicinally orientated oxygen atoms in the P(V)-TBP state (Scheme I). It has been emphasized regularly'8~b,hthat this concept of conformational transmission forms an effective mechanism by which the conformation of phosphorylated biomolecules possessing the P-O-C-C-0 atomic sequence, e.g. RNA, DNA and phospholipids, can be changed. In a previous paper: we reported a variable-temperature 13C NMR study on a series of monocyclic oxyphosphoranes in order to determine the influence of this conformational transmission effect on the barriers to pseudorotation. It was demonstrated, by examining compounds 1 and 2, that the pseudorotation rate of monocyclic oxyphosphoranes lb, Id, 2b, and 2d, which exhibit the conformational (1) (a) Koole, L. H.; Lanters, E. J.; Buck, H. M. J. Am. Chem. SOC. 1984,106, 5451. (b) Koole, L. H.; van Kooyk, R. J. L.; Buck, H. M. J. Am. Chem. SOC. 1986,107,4032. (c) De Vries, N. K.; Buck, H. M. Red. Trav. Chim. Pays-Bas 1986,106,150. (d) De Vries, N. K.; Buck, H. M. Phosphorw Sulfur 1987,31,267. (e) Van Genderen, M. H. P.; Koole, L. H.; Olde Scheper, B. G. C. M.; van de Ven, L. J. M.; Buck, H. M. Phosphorus Sulfur 1987, 32,73. ( f ) Van Genderen, M. H. P.; Buck, H. M. Red. Trav. Chim. PQYS-BQS 1987,106,449. (9) Van Genderen, M. H. P., Buck, H. M. Magn. Reson. Chem. 1987,25,872. (h) Meulendijks, G. H. W. M.; van Es, W.; de Haan, J. W.; Buck, H. M. Eur. J. Biochem. 1986, 157, 421. (2) (a) Holmes, R. R. Pentacoordinated Phosphorus; American Chemical Society: Washington, DC, 1980; Vol. 1 and 2 (ACS Monograph no. 175 and 176). (b) Luckenbach, R. Dynamic Stereochemistry of Pentacoordinated Phosphorus and Related Elements: George Thieme Verlag: Stuttgart, 1973. (3) De Keijzer, A. E. H.; Koole, L. H.; Buck, H. M. J.Am. Chem. Soc., in press.
0022-3263/88/1953-4827$01.50/0
Scheme I
0
A
Nu
Scheme 11 R
R
R
do J transmission effect, is 2-4 times faster as compared to that of their counterparts la, IC, 2a, and 2c in which no conformational transmission occurs. A straightforward explanation for the lowering of the pseudorotation barriers in the compounds with X = 0 presented. It was shown that the interconversion of the ground-state T B P in the pseudorotation process proceeds via one T B P and two square-pyramidal (SP) transition states. Furthermore, it was demonstrated that in these SP structures, which determine the magnitude of the activation barriers in the ligand exchange process, a net stabilization due to the conformational transmission effect occurs. As a result, a lowering of the activation barrier by 2-3 kJ mol-' was found. FH3 n
la : X=CH2,Rl=H,R2=C6H5 lb : X=O,R1=H,R2=C6H5 IC : X=CH2,Ri=Rz=CH3 Id : X=O,Ri=R2=CH3
2a : X = C H 2 , R i = H , R 2 = C 6 H 5 , R 3 = C H 3 2b : X=O,Ri=H,R2=C6H5.R3=CH3 2c : X = C H Z , R ~ = R ~ = C H ~ , R ~ = O C ~ H ~
2d : X = O , R ~ = R ~ = C H ~ , R ~ = O C ~ H S
In addition it was briefly noted that an alternative mechanism. in which a hexacoordinated zwitterionic 1988 American Chemical Society
4828 J. Org. Chem., Vol. 53, No. 20, 1988
de Keijzer and Buck
Table I. Calculated Proton-Proton Coupling Constants (Hz) for the Rotamers in Compounds 5-8 g+
5a,b 6a,b 7a,b 8a,b
g‘
JW
53’6’
1.76
535”
J3,5,
1.74
3.95
J1’2’
53‘5’
12.61
12.61
phosphorus transition state might account for the more rapid ligand reorganization rates, as was proposed earlier by Eisenhut et al.4 in case of the solvent-induced acceleration of pseudorotation in (CH3)2NPF4,is most unlikely. We now present a detailed study on the synthesis, conformational analysis, and 13C NMR variable-temperature experiments of several new monocyclic oxyphosphoranes. A careful examination of the selected compounds will enable us to discriminate between the two mechanisms, conformational transmission or hexacoordinated zwitterionic phosphorus transition states, concerned. The invocation of a mechanism that involves hexacoordinated intermediates would require the addition of one of the additional ligand oxygens t o the central phosphorus atom to form a bicyclic zwitterionic hexacoordinated intermediate. A subsequent ring opening, accompanied with a slight movement of the ligands to form a new trigonal plane, then results in a Berry permutation5 as is generalized in Scheme 11. The most straightforward mechanism is presented, a number of other mechanisms can be drawn that would have the same permutational result as the one presented. In order to investigate the possibility of the acceleration of pseudorotation by means of such a hexacoordinated intermediate, in contrast to the mechanism involving conformational transmission, a number of new compounds have been synthesized. Of special interest are compounds 3a, 3b, 4c, and 4f, which show no conformational transmission but possess additional oxygen atoms in the ligands, thus permitting a zwitterionic transition state to accelerate the pseudorotation rate. n
5351,
J12,
3.97
4.82 4.11 4.93
4.82 4.11 4.93
CH3
gJ31y
7.92 7.50 8.12
tion of the C3,-C5, conformation in compounds 3a and 3b and the C,,-C2, conformation in compounds 4a-f was hampered by decoalescence phenomena. We therefore selected the closely related model compounds 5a-8b, upon which the conformational analysis was performed. The C3,-Csrand C1.-C2, conformations of compounds 5a, 5b and 6a-8b, respectively, are based on the modified Karplus relationship as developed by Haasnoot e t al.6 This Karplus7 equation relates the vicinal coupling constants of an ethane fragment to the torsion angle between the coupling protons.
y_Jo 5a 5b
y-43
0
6 a : X=Y=CH2 7a : X=O,Y=CH2 8a : X=CHz,Y=O
6b : X=Y=CHz 7b : X=O,Y=CH2 8b : X=CHz,Y=O
This standard equation was extended with a correction term that accounts for the influence of electronegative substituents on J H H : J ( H H ) = P, cos2 4
+ P2cos 4 + P3+ CAXi[Pd i -I- ps
COS2
(ti4
P6lAXil)l
AX, = AX? - P7CAX; J -
4c 4d 4e 4f
.
-
. .
.
.
: X=CH2,Y=O,R1=H,R*=C6H5,R3=CH3 : X=Y=CH~,RI=R~=CH~,R~=OC~H~ : X=O,Y=CH2,Rl=R2=CH3,R3=OC2H5 :
X=CH2,Y=O,R1=R2=CH3,R3=OC2H5
The isomerization processes of these new compounds were followed by variable-temperature 13CNMR studies, and the activation barriers of the pseudorotation processes were determined. Comparing these barriers with those of the phosphoranes in which conformational transmission is present (4b, 4e) or absent (4a, 4d), respectively, allows us to draw some conclusions about the possible involvement of a hexacoordinated phosphorus intermediate.
In this equation, 4 is the proton-proton torsion angle, AXi is the difference in electronegativity between the a-substituents and hydrogen according to the electronegativity scale of Huggins,s corrected for @-substituents,and ti is a substituent orientation parameter. Values of P1-P7used for both cyclic and acyclic (in parentheses) systems are: Pi = 13.22 (13.89), Pz = -0.99 (4.98), P3 = 0 (O), P4 = 0.87 (1.02), P 5 = -2.46 (-3.40), P6 = 19.9 (14.9), P7 = 0 (0.24). The theoretical values of J3t5t and J3t5”of compounds 5a and 5b, as well as the values of JIJzf for compounds 6a, 6b, 7a, 7b, 8a, and 8b have been calculated for each staggered rotamerg and are collected in Table I.
Results and Discussion Conformational Analysis. The accurate determinaEisenhut, M.; Mitchell, H. L.; Traficante, D. D.; Kaufman, R. J.; Deutch, J. M.; Whitesides, G. M. J . Am. Chem. SOC.1974, 96, 5385. (5) Berry, R. S. J . Chem. Phys. 1960, 32, 933. (4)
(6) Haasnoot, C. A. G.; de Leeuw, F. A. A. M.; Altona, C. Tetrahedron 1980, 36, 2783. (7) Karplus, M. J. Chem. Phys. 1959, 30, 11. (8) Huggins, M. J. J . Am. Chem. SOC.1953, 75, 4123.
J. Org. Chem., Vol. 53, No. 20, 1988 4829
Pseudorotation Rates in Phosphorus Compounds
Table 11. Measured Proton-Proton Coupling Constants and Calculated Rotamer Populations in Compounds 5-8O JHw
5a 5b 6a 6b 7a 7b 8a 8b
JHH
JHH"
"gt.
Xgt
Xg-
7.6 7.7
0.35 0.36
0.47 0.47
0.18 0.17 0.58
6.6 6.7 6.6 6.7 4.9 5.5 6.4 6.4
0.42 0.40 0.78 0.60 0.55 0.55
0.60
0.22 0.40 0.45 0.45
The rotamer populations are uncorrected for phosphorus pseudorotation.
Table 111. Activation Parameters for the Exchange Processes in Phosphoranes 1,3, and 4 R, R, R" RA" T,b AuC AG,' H C6H5 CH3 CP 288e 325e 54.6 270e 295' 51.2 C6H5 CH3 THFF H 294 f 408f 55.3 CH3 THFM C6H5 H
la lb 3a 4a 4b 4c
H H H
C6H5
IC
Id 3b
CH3 CH3 CH3
CH3 CH3 CH3
OCZH5
4d 4e 4f
CH3 CH3 CH3
CH3 CH3 CH3
OCZH5 OCZH5 OCZH5
C6H5 C6H5
CH3 CH3 CH3 CH3
(3%
A
222 199 215
51.7 50.1 52.0
B
C~HGOCH,
269 260 271
CP THFF THFM
371e 358' 365
285e 216e 165
71.5 69.7 71.9
C
C5H11
325 316 323
205 186 196
63.2 61.6 62.9
D
C5Hll C2H40C2H5
C2H40C2H5
C3HeOCH3
CP = cyclopentanemethyl, THFF = tetrahydrofurfuryl, THFM = 3-tetrahydrofuranylmethyl. Solvents: group A and B, C6D5CD3igroup C and D, CBD5Br.bThe coalescence temperatures, T,(K) refer to the temperatures of maximum broadening of the NMR signals studied and were determined with an accuracy of f 2 K. CDifferencesin chemical shifts (Hz) between the equatorial and axial sites in the absence of exchange, measured with an accuracy of f 2 Hz. AGc* values (kJ mol-') calculated from the equation AG' = (1.91 X 10-2)Tc(9.973+ log (TJAu)). Estimated uncertainty f0.4 kJ molv1. e T,and Au of compounds la-d have been taken from ref 3. f T,and Au of compound 3a have been determined from the low-temperature 100-MHz I3C NMR spectrum of this compound.
The population densities for the individual rotamers can now be obtained by using the experimental parameters J W and the theoretical values of Jgt,J k and Jg-in the equation: with the normalization equation: xg+ Xgt xg- = 1
+ +
The spectral parameters for compounds 5a and 5b were taken from the 300-MHz expansion plots of the H5t5tt patterns and iteratively analyzed with the PANIC program.1° The coupling constants Jlr2, of compounds 6a, 6b, 7a, 7b, 8a, and 8b were determined from the 200-MHz expansion plots by employing the same standard computer simulation-iteration procedure. The correct assignment of the patterns in the expansion plots of the rather complex (9) In solution a rapid interconversion between the three staggered conformations g+, gt, and g- yields weighted time-averaged coupling constants J3a8and which are related to the individual rotamers and their populations. J3T5t,,
-
L'
HL'
9+
91
g-
In case of the acyclic compounds, two of these rotamers (g+,gt) are mirror images and have identical populations, therefore a two-state description with a gauche and a trans state is used.
-2
H2
gauche
H2 Irons
(IO) PANIC program, copyright Bruker Spectrospin AG, Switzerland.
H,,1,,/H4,4tl/H5r5,, region was determined from the two-dimensional J-resolved 300-MHz 'H NMR spectrum of both the P(1V) and P(V) compounds 5a and 5b and the precursor alcohol. The individual assignment of H5,and H5" was arbitrarly chosen in line with the one used by Koole et al.la for the tetrahydrofurfuryl and cyclopentylmethyl compounds. A reverse assignment would only affect the g- and gt populations. The g+ population remains unchanged, the gt and g- populations interchange. Both assignments result in the same conclusion, i.e. no change in rotamer populations upon going from a P(1V) to a P(V) coordination. The spectral parameters determined for the P(1V) and P(V) compounds 5a-8b, along with the resulting rotamer populations, are listed in Table 11. From these data we may now conclude that no conformational transmission occurs in compounds 5b, 6b, and 8b. Applying these results to the compounds 3 and 4 means that conformational transmission is present in 4b and 4e and that no conformational transmission occurs in 3a, 3b, 4a, 412, 4d, and 4f. Exchange Process Studies. The POCH2 moieties of compounds 3 and 4 exhibit an exchange process that can be readily followed by variable-temperature 13C NMR analysis and allows the determination of the pseudorotation barriers. The activation parameters of the exchange process have been evaluated according to the method described p r e v i ~ u s l y .The ~ results of the I3C NMR variable-temperature studies on the compounds 1, 3, and 4 have been summarized in Table 111. From these data, and the data regarding the conformational transmission effect, we are now able to draw some conclusions concerning the possible involvement of hexacoordinated intermediates. Comparing the data of compound sets A (la, l b , and 3a), B (4a, 4b, and 4c), C (IC, ld, and 3b), and D (4d, 4e, and 4f), respectively, it is clear that, in all cases, the compounds exhibiting the confor-
4830 J. Org. Chem., Vol. 53, No. 20, 1988
'H CH3 COCH3 POCHZ" H4 Hl,/H,, HP' H2 H3, C(CH3)z COCHZ OCHZCH3 C6H5
X Y YCH3 31P a
de Keijzer a n d Buck
Table IV. 'H and slP NMR Data for the Phosphoranes 3 and 4 a t 25 "C 3a 3b 4a 4b 4c 4d 1.68 2.10 1.69 1.63 1.68 2.11 2.46 2.48 2.49 2.42 3.23-3.78 3.30-3.82 3.70 3.84 3.83 3.63 4.15 4.15 4.13 4.15 3.23-3.78 3.30-3.82 1.24 1.24 1.34 3.27 1.59 1.38 1.59 1.60 2.16 2.16 1.46 1.48 3.93 3.93 0.99 1.00 6.94-7.24 6.93-7.23 6.94-7.28 6.95-7.22 1.15 3.18 1.15 1.15 3.20 1.15 0.77 0.97 3.08 0.71 -30.3 -27.0 -29.0 -28.9 -29.0 -26.0
4e 2.05
4f 2.09
3.78
3.73
3.28
1.63
1.42 3.93 0.91
1.40 3.93 1.01 3.21
3.18 0.89 -25.9
3.02 -26.0
The POCH, signals of compounds 3 and 4 were broadened by slow exchange at room temperature.
mational transmission effect show a lowering of the pseudorotation barrier as compared to their counterparts in which the conformational transmission effect is absent, as could be expected on the basis of our previous investigations. Interestingly, however, the compounds 3a, 3b, 4c, and 4f in which no conformational transmission occurs, but which still possess additional oxygen atoms in the ligands thus allowing hexacoordinated zwitterionic intermediates to be formed, show virtually identical activation energies as those of the compounds in which no additional oxygen atoms a r e present. On the basis of these experimental results and the theoretical considerations presented in the previous paper,3 it is now justifiable to conclude that no hexacoordinated zwitterionic intermediates are formed in the monocyclic oxyphosphoranes studied. Therefore, the lowering of the p s e u d o r o t a t i o n barriers is entirely b r o u g h t about b y the presence of the conformational transmission effect. Conclusion This study clearly demonstrates the impact of the conformational transmission effect on the barriers to pseudorotation in monocyclic oxyphosphoranes. It clearly shows that the lowering of the activation energies is entirely based on the presence of the conformational transmission effect and that there is no involvement of a hexacoordinated zwitterionic intermediate in this type of monocyclic oxyphosphoranes. Experimental Section Spectroscopy. 'H NMR spectra of compounds 6-8 were run in the FT mode on a Bruker AC-200 instrument a t 200.1 MHz. For compound 5 as well as the 3-tetrahydrofuranmethanol, the one- and two-dimensional spectra were run on a Bruker CXP-300 spectrometer at 300.1 MHz. Proton chemical shifts are referenced against TMS as internal standard (6 0). ,'P NMR spectra were recorded in the FT mode a t 80.9 MHz on a Bruker AC-200 instrument. Chemical shifts are related to 85% as external standard and are designated positive if downfield with respect to the reference. 13CNMR spectra were run in the FT mode at 50.3 MHz on a Bruker AC-200 spectrometer (compounds 3b and 4) or a t 100.3 MHz on a Bruker AM-400 instrument (compound 3a). Chemical shifts are referenced against internal TMS. The variable temperature experiments were performed and analyzed as described previ~usly.~ All spectra were recorded in CDCl, unless stated otherwise. Synthesis. All solvents and commercial reagents were reagent grade and were dried prior to use with the appropriate drying agents. All moisture-sensitive compounds were handled under a dry nitrogen atmosphere. The general instability of the phosphites and oxyphosphoranes has precluded the obtention of
correct standard analytical data. The identification of these compounds rests therefore on 'H, 13C, and 31P spectroscopy, methods of preparation, and comparison of the obtained physical data with those presented for well-defined P(II1)and P(V) compounds." 3-Tetrahydrofuranmethanol. This compound was prepared form triethyl 1,1,2-ethanetricarboxylateaccording to literature procedures.12 Bp: 76-77 "C (4 mm). Yield: 37%. 'H NMR: 6 1.65 (m, 1 H, Hz), 2.04 (m, 1H, HrJ, 2.48 (m, 1H, H3,),3.40-3.70 (m, 3 H, H4,,/H5,/H5,,), 3.70-3.93 (m, 3 H, H4,/H1,/HlPr), 4.48 (s, 1 H, OH). 13CN M R 6 28.3 (Cz,), 41.1 (C3,),63.9 (C40, 67.4 (Ci,), 70.2 (C5,). Anal. Calcd for C5Hl0OZ:C, 58.80; H, 9.87. Found: C, 58.62; H, 9.59. MS calcd for C5HlOOz m / e 102.13, measured 102.15. 3-Methoxy-1-propanol. This compound was prepared from 1,3-propanediol according to a literature p r 0 ~ e d u r e . l ~ Bp: 160-168 "C. Yield: 69%. 'H NMR: 6 1.70 (m, 2 H, CH2), 3.23 (s, 3 H, OCH,), 3.40 (t, 2 H, CHzO), 3.55 (s, 1 H, OH), 3.57 (t, 2 H, OCHJ. 13CNMR: 6 31.8 (CHz),58.2(CHzOH),59.8(OCH,), 70.2 (CH20). Anal. Calcd for C4HloOz: C, 53.31; H, 11.18. Found C, 53.49; H, 11.09. Phosphites. All phosphites were prepared from the corresponding alcohols and PCl, according to the procedure described for the preparation of tris(cyc10pentylmethyl)phosphite.~ T r i p e n t y l Phosphite. Bp: 83 "C (0.04 mm). Yield: 60%. 'H NMR: 6 0.91 (t,9 H, CH,), 1.36 (m, 12 H, CHzCHz),1.62 (m, 6 H, OCH2CHz),3.80 (dt, 6 H, POCHJ. 13C NMR: 6 13.8 (CH,), 22.2 (CH,CH,), 27.9 (CH,), 30.8 (OCHJH,), 62.1 (POCHJ. 31P N M R 6 139.8. Tris(2-ethoxyethyl) Phosphite. Bp: 102 "C (0.07 mm). Yield: 74%. 'H NMR: 6 1.20 (t, 9 H, CH3), 3.52 (t, 6 H, OCHZCH,), 3.58 (t, 6 H, CHZCHZO), 3.97 (dt, 6 H, POCHZ). 13C NMR: 6 14.8 (CH,), 61.1 (POCHZ), 66.1 (OCHZCH,), 69.9 (CH2CH20). ,'P NMR: 6 140.1. Tris(3-methoxypropyl) Phosphite. Bp: 101 "C (0.01 mm). Yield: 44%. 'H N M R 6 1.89 (m, 6 H, CIIz), 3.32 (s,9 H, OCH,), 3.47 (t, 6 H, CH,O), 3.90 (dt, 6 H, POCHz). 13C NMR: 6 30.9 (CH2),58.1 (OCH3),58.7 (POCH,), 68.6 (CHzO). ,'P NMR: 6 139.5. Tris(3-tetrahydrofuranylmethyl)Phosphite. Bp: 154-156 "C (0.005 mm). Yield: 47%. 'H NMR: 6 1.24 (m, 3 H, Hy), 1.54 (m, 3 H, Hz,,),2.16 (m, 3 H, H,,), 3.27-3.72 (m, 18 H, Hl,,l,,, H4, 4N, POCHJ. 13C NMR: 6 30.0 (Cz,),41.7 65.1 (C5,),68.5 71.4 (C4,). 31PNMR: 6 139.0. Phosphates. All phosphates were obtained by oxidation of the corresponding phosphites. An ozone oxygen stream was passed through a solution of the phosphite in dry dichloromethane at
(d,,),
(11) (a) Ramirez, F.; Madan, 0. P.; Heller, S. R. J. Am. Chem. SOC. 1965,87,73. (b) Gorenstein, D.; Westheimer, F. H. J. Am. Chem. SOC. 1970,92,634. ( c ) Buono, G.; Llinas, J. R. J . Am. Chem. SOC.1981,103, 4532. (d) Aganov, A. V.; Polezhaeva, N. A,; Khayanov, A. I.; Arbuzov, B. A. Phosphorus Sulfur 1985, 22, 303. (12) Bogner, J.; Duplan, J.-C.; Infarnet, Y.; Delmau, J.; Huet, J. Bull. Soc. Chim. Fr. 1972, 9, 3616. (13) Smith, L. I.; Sprung, J. A. J . Am. Chem. SOC.1943, 65, 1276.
J. Org. Chem., Vol. 53,No. 20,1988 4831
Pseudorotation Rates in Phosphorus Compounds
13c
3a 17.9 30.5 68.8 29.8 41.3 71.4 70.2 166.2 114.4 50.2
127.9-130.5 139.0 194.6
Table V. 13C NMR Data for the Phosphoranes 3 and 4 at 25 O C 3b 4a 4b 4c 4d 17.0 67.8 29.0 40.9 70.2 66.9 (a) 70.2 (e) 163.0 116.0 45.5 14.5 60.5 23.0 165.8
4f
18.1 30.3
18.0 30.3
18.0 30.5
17.4
4e 17.4
17.4
31.9
71.2
32.5
30.9
70.5
31.7
68.4
67.7
65.6
166.8 113.9 50.2
166.5 113.9 49.9
166.9 114.2 50.5
127.6-130.3 139.9 194.2 29.4 23.7 15.1
127.6-130.4 139.2 194.2
127.6-130.4 139.8 194.2 70.6
67.3 16.4
59.0
64.2 (a) 68.2 (e) 163.8 108.3 42.0 14.7 58.6 23.1
63.9 (a) 67.6 (e) 163.2 108.4 42.5 14.7 58.9 23.2
61.0 (a) 64.9 (e) 163.4 108.4 42.0 14.7 59.0 23.3
165.9 28.5 22.7 14.3
166.0
165.8 69.0
66.5 15.9
58.7
"Compounds 3b and 4d-f show no pseudorotation at 25 "C, signal intensities are approximately (a)/(e) = 1:2; (a) axial, (e) equatorial. 0 OC. After 1h the solution was sparged with oxygen and allowed to warm to room temperature. Evaporation of the solvent yielded the desired phosphates as was confirmed by ,'P NMR. Tripentyl Phosphate (sa). 'H NMR 6 0.90 (t,9 H, CH,), 1.37 (m, 12 H, CH2CH2), 1.70 (m, 6 H, OCH,CH,), 4.04 (dt, 6 H, POCH,). 13C NMR: 6 13.7 (CH,), 22.0 (CH,CH,), 27.4 (CH,), 29.8 (OCHzCH2),67.5 (POCH,). 31PNMR 6 -0.2. Anal. Calcd for C16H3,04P: C, 58.44; H, 10.71. Found: C, 58.53; H, 10.64. Tris(ethoxyethy1) Phosphate (7a). 'H NMR 6 1.21 (t, 9 H, CH,), 3.56 (q,6 H, OCH2CH3),3.64 (m, 6 H, CH,CH,O), 4.22 (m,6 H,POCH2). 13CNMR 6 14.7 (CH3),66.2 (OCH2CH3),66.4 (POCH,), 68.8 (CH2CH20). 31PNMR: 6 -0.5. Anal. Calcd for Cl2HZ7O7P: C, 45.86; H, 8.60. Found: C, 45.67; H, 8.73. Tris(3-methoxypropyl) Phosphate (sa). 'H NMR: 6 1.94 (m, 6 H, CH2), 3.36 (s, 9 H, OCH,), 3.48 (t,6 H, CHzO), 4.14 (dt, 6 H, POCH,). 13CNMR 6 30.0 (CH,), 58.2 (OCH,), 64.5 (POCHJ, 68.0 (CHzO). ,P ' NMR 6 -0.5. Anal. Calcd for ClzHn07P: C, 45.86; H, 8.60. Found: C, 45.83; H, 8.49. Tris(3-tetrahydrofuranylmethyl)Phosphate (5a). 'H NMR 6 1.23 (m, 3 H, H,.), 1.55 (m, 3 H, Hzt,), 2.21 (m, 3 H, H3J, 3.40 (m, 12 H, Hlj/lt,, H4t/4tf),3.68 (m, 6 H, POCH,). 13C NMR 6 28.7 (Czt), 40.1 (C39), 67.7 (Ci,), 69.4 (POCH,), 70.2 (C4,). 31P NMR: 6 0.0. Anal. Calcd for C15H2,07P: C, 51.42; H, 7.77. Found: C, 51.83; H, 7.83.
Phosphoranes. All phosphoranes used for the conformational analysis study were obtained by adding an equimolar amount of 2,3-butanedione at 0 "C to a solution of the corresponding phosphite in a deuteriated solvent. 31PNMR showed the reactions to be complete after 30 min at room temperature. 2,2,2-Tripentoxy-4,5-dimethyl1,3,2-dioxaphosphol-4-ene (6b). 'H NMR: 6 0.91 (t, 9 H, CH,), 1.38 (m, 12 H, CH,CH,), 1.59 (m, 6 H, OCH2CHz), 1.91 (s, 6 H, 2 CH,), 3.85 (m, 6 H, POCHJ. 13CNMR: 6 10.2 (CH,), 13.7 (CH,CH,), 22.2 (CH,CH,), 27.8 (CH,), 30.3 (OCH2CHz),67.1 (POCH,), 128.3 (C=C). NMR: 6 -50.4.
,'P
2,2,2-Tris(ethoxyethoxy)-4,5-dimethyl-l,3,2-dioxaphosphol-4-ene(7b). 'H NMR: 6 1.20 (t, 9 H, OCH2CH8), 1.83 (s, 6 H, 2 CH3), 3.53 (t, 6 H, OCH&HJ, 3.57 (t, 6 H, CH2CH20),
4.02 (m, 6 H, POCH2). 13CNMR 6 10.0 (CH,), 14.7 (OCH#H,), 65.8 (OCH&HJ, 66.0 (POCH,), 69.6 (CH&H20), 128.2 (C=C). ,'P NMR: 6 -50.4.
2,2,2-Tris(3'-methoxypropoxy)-4,5-dimethyl1,3,2-dioxaphosphol-4-ene(8b). 'H NMR 6 1.82 (s, 6 H, 2 CH3), 1.85 (m, 6 H, CH,), 3.31 (s, 9 H, OCH,), 3.48 (t, 6 H, CH,O), 3.94 (m, 6 13CNMR 6 10.2 (CH,), 30.5 (CH,), 57.8 (OCH,), 63.8 (POCH,), 68.9 (OCH,), 128.3 (C=C). ,'P NMR 6 -50.4.
H, POCH,).
2,2,2-Tris(3'-tetrahydrofuranylmethoxy)-4,5-dimethyl1,3,2-dioxaphosphol-4-ene(5b). 'H NMR (C6D5CD3): 6 1.66
(m, 3 H, H2,), 1.90 (s, 6 H, 2 CH,), 2.05 (m, 3 H, H2,,), 2.55 (m, 3 H, H30, 3.65-4.20 (m, 18 H, H1,',,, H4,/4t,,POCH,). 13CNMR (C6D&D3): 6 11.4 (CH,), 29.7 (&), 41.5 (C,,), 68.6 (C1,), 70.3 (POCH,), 71.6 (Cg), 130.2 (C=C). ,P ' NMR (CBD5CD3): 6 -51.0.
In order to avoid decomposition during handling and purification of the phosphoranes used for the variable temperature studies, they were prepared in situ in the NMR tubes by adding equivalent amounts of freshly distilled phosphite and the appropriate precursor pentadiones, to the deuteriated solvents. The tubes were flushed with argon and sealed. After leaving them at room temparure for 10-14 days, the reactions were complete as was confirmed by ,'P NMR spectroscopy. 'H, 13C, and 31P NMR spectra were then recorded and are listed in Table IV and V.
Acknowledgment. This investigation has been supported in part by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). The contribution of Dr. L. H. Koole in valuable discussions regarding the choice of the model compounds is gratefully acknowledged. Furthermore, we would like to thank Dr. N. K. de Vries of DSM Research Geleen, The Netherlands, for recording the variable temperature spectra of compound 3a, and Dr. J. W. de Haan and L. J. M. van de Ven for assistance in recording the 2D NMR spectra.
